1. Field of the Invention
The present invention relates to an intrusion alarm system. In particular, the invention relates to a vehicle intrusion alarm system which is attached to, and works in conjunction with, a building's alarm system.
2. The Prior Art
Many vehicles carry valuable cargo, ranging from tools to products (e.g., plumber's tools, television repairman's equipment, salesman's samples, telephone equipment, etc.); and in many cases, it is impracticable to unload the vehicle every night.
It is becoming increasingly more difficult to prevent pilferage from these parked vehicles and many schemes have been suggested for solving this problem. Unfortunately, most of these prior art schemes are complex, expensive, and subject to human error.
The present intrusion alarm system solves the above problems in a simple, fool-proof manner.
The following brief discussion of the prior art intrusion systems will provide an understanding of the problems and of the operation and advantages of the disclosed system. For ease of comprehension, it will be presented in terms of a truck parked on a parking lot, but it will be apparent that the disclosed invention has many other uses.
Many of the prior art intrusion systems provided an alarm when an intruder touched, approached, or passed a given barrier, such as a fence, but gave no alarm if the intruder somehow by-passed the barrier (as, for example, by being within the barrier when the intrusion system was turned on). In cases such as these, parked vehicles and the like could be pilfered at leisure without producing an alarm, the alarm being produced only when the intruder was already escaping with his loot. Some other intrusion systems produced an alarm when a body moved within a predetermined area, but due to the required sensitivity of such a system, the protected area had to be quite small. Other intrusion systems produced an alarm when a vehicle was driven across a given sensor, such as a wire, a lightbeam, an air hose, or the like, but such systems did not give any warning that pilferage was impending. Still other systems had alarms attached to the vehicle proper, the concept being that whenever the vehicle was jarred, the alarm would sound.
U.S. Pat. No. 3,688,256 to D'Ausilio et al. discloses a vehicle intrusion alarm system. The system comprises a transmitter located in each protected vehicle, and a receiver located around the perimeter of a protected area. When an intrusion in a protected vehicle occurs, the transmitter produces a varying magnetic field which is sensed by the receiver. The receiver then generates an alarm signal which either sounds an audible alarm or triggers an automatic dialer. This system has several disadvantages. First, at the start of a business day, the system must be deactivated so that the truck operators are able to enter the vehicles without sounding an alarm. This requirement leaves all the other vehicles unprotected. Second, any similar transmitter disposed outside of the receiver will similarly produce a false alarm. Lastly, the receiver requires a series of pick up loops disposed around the perimeter of a protected area. If the protected area is the parking lot of a large building, this requirement may add significantly to the cost of the system.
U.S. Pat. No. 4,125,826 to Rasmussen et al. discloses an ultrasonic vehicle alarm system. The system comprises a transmitter of ultrasonic energy and a receiver capable of receiving the same. An alarm occurs when either the transmitter is disabled due to an intrusion or the protected device is moved away from the receiver. A major drawback of this system is the requirement for the continual transmission of ultrasonic energy. This requirement draws power from the protected vehicle and is an unnecessary drain on the electrical system.
It is apparent that the available intrusion systems are not completely satisfactory.